The present invention relates to solar batteries for converting optical energy into electrical energy and, more particularly, to a bypass-function added solar cell to which is added a bypass-diode function for protecting the solar cell from reverse bias voltage.
Generally, solar cells are used as a solar cell module in which a plurality of solar cells are combined together in series and parallel.
In this solar cell module, when part of the cells are shadowed, voltages generated by other cells are applied to these cells in reverse directions.
For example, in space solar cell modules, there can occur a shadow of part of the satellite body or structures such as antenna onto the solar cell module during the posture control of the satellite. Also, in ground solar cell modules, for example, shadows of neighboring buildings can occur or shadows of attached droppings of birds that have come over flying.
As an example, here is discussed a case where there has occurred a shadow onto part of partial submodules of a solar cell module which is made up of an array of parallel-connected solar cells.
Referring to FIG. 9A, in a shunt mode in which both ends of a solar cell module M are nearly short-circuited, a voltage V12 generated by unshadowed other groups of submodules 312 is applied as a reverse bias voltage to a shadowed submodule 311. If the voltage of this submodule 311 is V11, then
V11=xe2x88x92V12.
As shown in FIG. 9B, when an external power supply VB is connected to a solar cell module M, it follows that V11=VBxe2x88x92V12. That is, a positive voltage is applied to an N electrode of the shadowed submodule 311, where if the reverse bias voltage of the voltage is higher than the breakdown voltage of the solar cells constituting the submodule 311, the cells would break down, causing a large amount of current to flow. In this case, if a crystal defect or the like is present in a cell, the current concentrates at the place, which may lead to a short-circuit breakdown of the cell occasionally. When this occurs, the shadowed submodule and further the entire solar cell module M are deteriorated in output characteristic.
In order to prevent accidents due to the application of this reverse bias voltage, bypass diodes are attached every solar cell or every particular module units, or so-called diode-integrated solar cells in which bypass diodes are integrated on solar cells are used.
Otherwise, there have been provided solar cells with the bypass diode function added. The structure of a bypass-diode-function added solar cell according to the prior art (Japanese Patent Laid-Open Publication HEI 8-88392) is described below with reference to FIGS. 4A-4C. FIG. 4B is a plan view showing the structure of this solar cell, and FIG. 4C is a sectional view taken along the line 4C-4Cxe2x80x2 of FIG. 4B. In this solar cell, an electrically conductive region for adding a bypass diode function is provided just under the light-receiving side electrode, thus the solar cell being equipped with the bypass-diode function is obtained without reducing the effective area of the light-receiving surface of the solar cell.
As shown in FIG. 4C, a light-receiving surface on top of a silicon p-type substrate 101 is covered with a transparent antireflection film 108, and under the antireflection film 108, a comb-tooth like n electrode 107 branched from an n-electrode connecting portion 105, which is a bar electrode, is placed on an n-type region 102 on top of the p-type substrate 101. Also, as shown in FIGS. 4B and 4C, a plurality of island-like p+ type regions 104 are provided just under the light-receiving electrode 107 with an insulating film 109 interposed therebetween, by which the function of such a bypass diode D as shown in FIG. 4A is added.
For this solar cell, as shown in FIG. 5B, oxide 110 is formed on the p-type substrate 101 shown in FIG. 5A. Then a plurality of openings 114 are formed in this oxide 110 as shown in FIG. 5C, and a p+ impurity is injected thereinto, by which the island-like p+ type regions 104 are formed as shown in FIG. 5D. Next, as shown in FIG. 5E, the n-type region 102 is formed on the top and side surfaces of the p-type substrate 101 by thermal diffusion or the like. Thereafter, as shown in FIG. 5F, the insulating film 109, the n electrode 107 and the n-electrode connecting portion 105 are formed and, further thereon, the antireflection film 108 and a rear-surface p electrode 106 are formed by vacuum deposition or the like. By cutting along both-side broken lines, the solar cell shown in FIG. 4C can be obtained.
This solar cell is connected in multiplicity in series and in parallel as shown in FIG. 9A so that desired voltage and current can be obtained. This product is used as the solar cell module M, generally.
In order to form the insulating film 109 on a plurality of island-like p+-type regions 104 as in the cross-sectional structure shown in FIG. 4C, after forming the p+-type regions 104, an insulating film 109 such as oxide is deposited by CVD process (Chemical Vapor Deposition Process) or the like all over the substrate surface. Thereafter, heat treatment such as RTA (Rapid Thermal Anneal) is required to compact this insulating film 109, and further a step of patterning the insulating film 109 into an island shape on the p+-type regions 104 is required. As a result, there has been a problem that the manufacturing cost becomes higher.
Further, a high-precision technique is involved in the patterning of the insulating film 109 in order that the p+-type regions 104 do not make contact with the light-receiving electrode 107. This poses another problem of complicated process.
Meanwhile, a manufacturing method including a steps of providing an electrically conductive type region for adding the bypass-diode function with the ion implantation process is described in Japanese Patent Laid-Open Publication HEI 5-110121. The structure of a solar cell with the bypass-diode function manufactured by this manufacturing method is shown FIG. 6 and FIGS. 7A, 7B and 7C.
In a bypass-diode function added solar cell as shown in FIGS. 7A and 7B, an n-type region 202 is formed on a p-type region 201 and a small island-like p+-type region 204 is formed in this n-type region 202. Further, an n-type electrode connecting portion 205 is formed on this n-type region 202. In a solar cell shown in FIG. 7C, an island-like p+-type region 204 is formed in a p-type region 201, and an n-type electrode connecting portion 205 is formed on the n-type region 202.
With regard to this solar cell, first, as shown in FIG. 8B, oxide film 209 is formed by thermal oxidation or the like all over a silicon p-type substrate 201 shown in FIG. 8A and then, as shown in FIG. 8C, a plurality of openings 214 are formed in the oxide film 209. Next, a p-type impurity is implanted into the p-type region 201 with the oxide film 209 used as a mask, and thereafter the oxide film 209 is removed, by which an island-like p+-type region 204 is formed on top of the p-type substrate 201 as shown in FIG. 8D. Next, as shown in FIG. 8E, the n-type region 202 is formed by thermal diffusion or the like on the top, bottom and side surfaces of the p-type substrate 201, and further an n electrode connecting portion 205 shown in FIG. 8F and an n electrode 207 shown in FIG. 6 are formed, and thereafter an antireflection film 208 and a rear-surface p electrode 206 are formed by vacuum deposition or the like. Finally, by cutting along both-side broken lines, a solar cell having a structure shown in FIGS. 6 and 7 can be fabricated.
However, in the solar cell shown in FIGS. 7A and 7B, there is a problem that the presence of the small island-like p+-type region 204, which is an electrically conductive region for adding the bypass-diode function as shown in FIG. 6, causes a reduction in the effective area of the light-receiving surface of the solar cell.
Also, in the solar cell shown in FIG. 7C, since the p+ region 204 is present at the pn junction interface, which is important for photoelectric conversion, minority carriers generated in the p region 201 are inhibited from being effectively collected to the n region. This causes a problem of deteriorated electrical output characteristics.
In view of these and other problems, an object of the present invention is to provide a bypass-function added solar cell, as well as a method for manufacturing the solar cell, which makes it possible to manufacture a solar cell with an effective bypass-diode function added thereto, with low cost and by simpler process and without reducing the effective area of the light-receiving surface of the solar cell.
In order to achieve the above object, there is provided a bypass-function added solar cell comprising:
a first-conductive-type first region;
a second-conductive-type second region formed on a light-receiving surface side of the first-conductive-type first region;
a first-conductive-type third region which is formed at part of a pn junction plane where the first region and the second region abut on each other so that the third region is stretched over the first region and the second region, that is, projected into both the first region and the second region, the third region being higher in impurity concentration than the first-conductive-type first region.
In this invention, the third region is formed at the boundary of the first-conductive-type first region and the second-conductive-type second region so as to be separated away from the surface of the light-receiving surface side surface of the second region. Therefore, in this invention, unlike the prior-art counterparts, the insulating film for isolating the first-conductive-type third region constituting the diode and the electrodes formed on the light-receiving surface side surface of the second region from one another is no longer necessary. Therefore, the solar cell of this invention, with a bypass-diode function added thereto, can be manufactured without reducing the effective area of the light-receiving surface, with low cost and by simple process.
In one embodiment of the present invention, a plurality of the first-conductive-type third region are provided.
In this embodiment, since a plurality of first-conductive-type third regions are provided, it follows that a plurality of np diodes exerting the bypass function can be contained. Thus, the reverse current can be distributively passed so that the possibility of partial breakdown due to current concentration can be reduced.
In one embodiment of the present invention, the light-receiving surface side electrode in abutment on part of the second region is formed just above the third region.
In this embodiment, since the light-receiving surface side electrodes are formed just above the third regions, third regions that cannot effectively convert light are preparatorily contained in the shadow portion due to the light-receiving surface side electrodes. Therefore, the solar cell as a whole can exert photoelectric conversion effectively.
In one embodiment of the present invention, in the ion implantation step, ion implantation is performed with a photosensitive resin used as a masking material to thereby form the third region formed into an island shape.
In this embodiment, since the first-conductive-type third regions are distributed in a dotted or linear shape, the bypass-diode function can be distributed efficiently to a broad area.
Also, there is provided a method for manufacturing the bypass-function added solar cell, comprising:
an ion implantation step of implanting ions into the first-conductive-type first region to thereby form the first-conductive-type third region higher in impurity concentration than the first-conductive-type first region at part of a pn junction plane of the first region and the second region so that the third region is projected into both the first region and the second region.
In this manufacturing method of this invention, by the ion implantation step of implanting ions into the first-conductive-type first region, the first-conductive-type third region higher in impurity concentration than the first-conductive-type first region is formed at part of the pn junction plane of the first region and the second region. As a result, the insulating film for isolating the third regions and the electrodes formed on the light-receiving surface side surface of the second region from one another is no longer necessary. Thus, a solar cell with the bypass-diode function added thereto can be provided with low cost and by simple process.
In one embodiment of the present invention, in the ion implantation step, any one of boron, gallium, aluminum and indium is used as a doping material.
In the manufacturing method of this embodiment, by the ion implantation step using any one of boron, gallium, aluminum and indium as a doping material, the first-conductive-type third regions can be formed at part of the pn junction plane of the first region and the second region so as to project into both the first region and the second region.
In one embodiment of the present invention, the method further comprises, after the ion implantation step, forming the second-conductive-type second region by thermal diffusion process and, simultaneously therewith, activating the third region.
In this manufacturing method of this embodiment, the second-conductive-type second region is formed by thermal diffusion process and, simultaneously therewith, activating the third region. Thus, the method becomes a highly efficient manufacturing method.
In one embodiment of the present invention, in the ion implantation step, ion implantation is performed with a photosensitive resin used as a masking material to thereby form the third region formed into an island shape.
In this embodiment, by performing ion implantation into the first region with a photosensitive resin used as a mask, the third region can be formed into a desired pattern.
In one embodiment of the present invention, in the ion implantation step, an ion beam controlled to a specified area is implanted to thereby form the third region.
In this embodiment, since ion implantation is done in a beamed manner by an ion implantation process with the beam throttled to a area over which the third region is formed, photosensitive resins are no longer necessary so that the manufacturing process of the solar cell can be more simplified.
Thus, with the solar cell manufacturing method of the present invention, a solar cell which is unlikely to be subject to the occurrence of short-circuit breakdown due to the reverse bias voltage can be manufactured with low cost. In particular, in the case of, for example, a space solar cell array which is difficult to maintain, remarkable effects can be produced for the protection against the reverse bias voltage, so that the reliability as a whole array can be improved. Also, since externally provided bypass diodes are not required, the manufacturing cost for the solar cell can be reduced.
Also, there is provided a bypass-function added multi-junction stacked type solar cell in which the bypass-function added solar cell of this invention is stacked, in a plural number as sub-cells, in series along a direction of incidence of light.
In the multi-junction stacked type solar cell of this invention, since a plurality of sub-cells are stacked in series along the direction of incidence of light, a high conversion efficiency can be achieved.
Also, the bypass-function added multi-junction stacked type solar cell of one embodiment includes the bypass-function added solar cell of this invention in which is the bypass-function added solar cell is stacked in a plural number as sub-cells in series along a direction of incidence of light and in which the light-receiving surface side electrodes in abutment on part of the second region are formed just above the third regions.
In the multi-junction stacked type solar cell of this embodiment, since a plurality of the sub-cells are stacked in series along the direction of incidence of light, a high conversion efficiency can be achieved. Also, since the bypass-function added solar cell in which the light-receiving surface side electrodes in abutment on part of the second region are formed just above the third regions is included, third regions that cannot effectively convert light are preparatorily contained in the shadow portion due to the light-receiving surface side electrodes. Therefore, the solar cell as a whole can exert photoelectric conversion effectively.
In one embodiment of the present invention, active layer portions of the solar cells as sub-cells are made of a group III-V compound semiconductor and the substrate is made from Ge or a group III-V compound semiconductor wafer.
In this embodiment, using group III-V compounds for the active layer part makes it possible to easily change the forbidden band width Eg and the lattice constant, and using Ge or a group III-V compound semiconductor for the substrate makes it possible to take lattice matching.
In one embodiment of the present invention, the number of third regions differ from sub-cell to sub-cell.
In this embodiment, since the number of third regions differs from sub-cell to sub-cell, a desired bypass ability can be set for each of the sub-cells according to the reverse I-V characteristics of the sub-cells in the dark state while suppressing any reduction in effective area.
In one embodiment of the present invention, a number of third regions formed in a top cell positioned closest to the light-receiving surface is the largest among the sub-cells.
In this embodiment, since the number of third regions formed in the top cell positioned closest to the light-receiving surface is the largest among the sub-cells, a bypass ability can be set so as to be ready for cases where a relatively large-area shadow occurs to this solar cell. The reason of this is that when a large-area shadow occurs, it is often the case that a reverse bias voltage is applied to the top cell.
In one embodiment of the present invention, a number of third regions formed in a sub-cell that is the smallest in production current density during photo-irradiation is the largest.
In this embodiment, since the number of third regions formed in a sub-cell that is the smallest in production current density during photo-irradiation is the largest, a bypass ability can be set so as to be ready for cases where light is irradiated to part of this MJ stacked type solar cell. The reason of this is that when a reverse bias voltage is applied with light impinging on part of the MJ cell, it is often the case that the reverse bias voltage is applied to a cell that is relatively small in photoproduction current amount.
In one embodiment of the present invention, positions on cell planes where the third regions are formed are uniform regardless of positions of the light-receiving surface side electrodes.
In this embodiment, since the positions on cell planes where the third regions are formed are uniform regardless of the positions of the light-receiving surface side electrodes, the bypass diode function can be distributed efficiently to a broad area.
In one embodiment of the present invention, positions on cell planes where the third regions are formed are positions under the light-receiving surface side electrodes.
In this embodiment, since the positions on cell planes where the third regions are formed are positions under the light-receiving surface side electrodes, third regions that cannot effectively convert light are preparatorily contained in the shadow portion due to the light-receiving surface side electrodes. Therefore, the solar cell as a whole can exert photoelectric conversion effectively.
In one embodiment of the present invention, among the individual sub-cells, positions where the third regions are formed on their cell planes are positions different from one another under the light-receiving surface side electrodes.
In this embodiment, among the individual sub-cells, the positions where the third regions are formed on their cell planes are positions different from one another under the light-receiving surface side electrodes. Therefore, the controllability of ion implantation depth for the formation of the third region can be improved.
In one embodiment of the present invention, the multi-junction solar cell of this invention further comprises the step of: after forming the third regions at positions under the light-receiving surface side electrodes, activating these third regions by beam annealing.
In this embodiment, since the third regions are activated by beam annealing, the rate of activation of implanted ions can be enhanced.
In one embodiment of the present invention, ion implantation material is one or a plurality of Be, Cd, Mg, Zn and C, or a combination of one or a plurality of Be, Cd, Mg, Zn and C and one of B, Al, Ga and In.
In this embodiment, it is effective to implant ions of Be, Cd, Mg, Zn or C for the formation of the p+ region as the third region in the group III-V compound semiconductor, and it is effective to implant ions of a group III element such as B, Al, Ga and In for the formation of the p+ region in the Ge substrate.
In one embodiment of the present invention, ion implantation material is one or a plurality of S, Se, Te and Si, or a combination of one or a plurality of S, Se, Te and Si and one of N, P, As and Sb.
This embodiment is effective for cases where the n+ region as the third region is formed in a solar cell having a xe2x80x9cp on nxe2x80x9d construction as viewed from the light-receiving surface side.
As apparent from the above description, according to the present invention, it becomes to add a desired bypass function to the MJ solar cell, contributing to reliability improvement as well as manufacturing cost reduction of the solar cell array using a high efficiency solar cell.